code: def test_ema_std():
      orig = np.array(
          [
              [
                  np.nan,
                  0.7071067811865476,
                  0.9746794344808964,
                  1.1143420667632726,
                  1.1785687889316867,
                  1.20612962779329,
                  1.217443715603457,
                  1.2219416913579804,
                  1.2236866244000921,
                  1.2243507269461653,
              ],
              [
                  np.nan,
                  0.7071067811865476,
                  2.2693611435820435,
                  4.280032864205755,
                  6.511621880314852,
                  8.846731940915395,
                  11.231335395956103,
                  13.640730921938678,
                  16.063365414263,
                  18.493615652686387,
              ],
          ]
      )
      dforig = pd.DataFrame({"0": orig[0], "1": orig[1]}).dropna()
      l1 = range(10)
      l2 = [i**2 for i in range(10)]
      df = pd.DataFrame({"0": l1, "1": l2}).astype(np.float64)
      res = ema_std(df, span=2).dropna()
      assert all((abs(dforig - res) <= 1e-15).all())
  
file_path: FinQuant/tests/test_moving_average.py
imports: import matplotlib.pyplot
  import numpy
  import pandas
  from finquant.moving_average import compute_ma, ema, ema_std, plot_bollinger_band, sma, sma_std
label_desc: Use image node_id 4 for calling a global function with example usage: test_ema_std() without return types